In our current implementation the implicit grid is only after the explicit grid.
For example, in a 2x2 grid, if you have an item with:
grid-row: 3;
An implicit 3rd row is created.
However, if you have an item like:
grid-column: -4;
We should be creating an implicit column before the first one.
This has been already implemented on Blink (http://crbug.com/444011)
following a plan described in this document (http://bit.ly/1j3CRdi).
We should port these patches to WebKit as the grid layout code has the same issues.
This will be used as meta-bug for all the patches related.
More info in the spec: http://dev.w3.org/csswg/css-grid/#line-placement